coronal hole

A coronal hole is a region of the Sun's corona that appears dark
in pictures taken with a coronagraph or during
a total solar eclipse, and that shows up as a void in X-ray and extreme ultraviolet images. Coronal holes are of very low density (typically
100 times lower than the rest of the corona) and have an open magnetic field
structure; in other words, magnetic field lines emerging from the holes
extend indefinitely into space rather than looping back into the photosphere.
This open structure allows charged particles to escape from the Sun and
results in coronal holes being the primary source of the solar
wind and the exclusive source of its high-speed component.

During the minimum years of the solar cycle,
coronal holes are largely confined to the Sun's polar regions (although
some exceptions have been observed by SOHO),
while at solar maximum they can open
up at any latitudes.

The magnetic fields from the center of coronal holes
in the Sun's atmosphere have large fluctuations known as Alfvén waves,
while those from the sides have smaller fluctuations. The side fields
do not transfer energy as well from the Sun to Earth's magnetosphere.
Image credit: NASA/Park